TITLE:   GCN CIRCULAR
NUMBER:  19355
SUBJECT: GRB 160425A: Swift-BAT refined analysis
DATE:    16/04/26 21:41:42 GMT
FROM:    Amy Lien at GSFC  <amy.y.lien@nasa.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J. R. Cummings (GSFC/UMBC),
N. Gehrels (GSFC), H. A. Krimm (GSFC/USRA), C. B. Markwardt (GSFC),
J. P. Norris (BSU), D. M. Palmer (LANL), T. Sakamoto (AGU), M. Stamatikos (OSU),
E. Troja (NASA/GSFC/UMCP), T. N. Ukwatta (LANL) (i.e. the Swift-BAT team):

Using the data set from T-240 to T+963 sec from the recent telemetry downlink,
we report further analysis of BAT GRB 160425A (trigger #684098)
(Krimm, et al. GCN Circ. 19343).  The BAT ground-calculated position is
RA, Dec = 280.339, -54.343 deg which is
  RA(J2000)  =  18h 41m 21.5s
  Dec(J2000) = -54d 20' 34.6"
with an uncertainty of 3.5 arcmin, (radius, sys+stat, 90% containment).
The partial coding was 71%.

The mask-weighted light curve shows several separate pulses. The first short pulse
starts at ~ T-0.5 s, peaks at ~T+0.2 s, and ends at ~T+1.5 s. The second pulse
occurs at ~T+60 s and is quite weak. Another strong pulse starts at ~T+250 s, peaks
at ~T+270 s, and ends at ~ T+300 s. T90 (15-350 keV) is 304.58 +- 15.04 sec
(estimated error including systematics). The potential precursor peak seen in the
raw light curve at ~T-380 s (Krimm, et al., GCN Circ. 19343) is due to a noisy
detector.

The time-averaged spectrum from T-0.53 to T+319.80 sec is best fit by a simple
power-law model. The power law index of the time-averaged spectrum is
2.20 +- 0.18. The fluence in the 15-150 keV band is 2.1 +- 0.2 x 10^-6 erg/cm2.
The total fluence is dominated by the pulse at ~T+270 s, which has a fluence of
1.1 +/- 0.1 x 10^-6 erg/cm2, while the fluence of the short pulse is
2.8 +/- 0.2 x 10^-7 erg/cm2. The 1-sec peak photon flux measured from
T-0.27 sec in the 15-150 keV band is 2.8 +- 0.2 ph/cm2/sec. All the quoted errors
are at the 90% confidence level.

The spectrum of the initial short pulse is best-fitted by a simple power-law model
with a power-law index of 1.71 +/- 0.14. This value is a typical number for a long
GRB, but is also consistent with the majority of the short GRB population
(Sakamoto et al. 2011).

Using a 32-ms binned light curve, the lag analysis of the initial short pulse
is 57 (-27, +34) ms for the 100-350 keV to 25-50 keV, and 19 (-38, +32) ms for
the 50-100 keV to 15-25 keV band. These numbers are consistent with
those of a long GRB. However, we note that the low redshift (z=0.555, Tanvir et al,
GCN Circ. 19350) indicates that this GRB might have a lower luminosity
(the luminosity of the short pulse in the BAT observed energy band, 15-150 keV,
is ~ 1.7 x 10^50 erg/s), and thus the this GRB might lie outside the usual
lag-luminosity relation (Norris et al. 2000).

The results of the batgrbproduct analysis are available at
http://gcn.gsfc.nasa.gov/notices_s/684098/BA/